nbmakefs 2G limit?

I am making NetBSD install ISO images that are very large. What I've
actually done is complicated, but the following is a reasonable
approximation.
turn on debugging for kernel and base system
build a bunch of packages (with -g)
export a git repo (500M worth)
dump the git repo and the packages in releasedir/i386 so they get put
on the iso image
This results in about 2.5G. disks are randomly not bootable, and I
can't read some of the files - as in 'cat /filename' gets some sort of
error return.). If we get rid of the git repo and get the contents down
to 2008 MB, then all seems well.
(sorry, I can't share the disk images. But I can perhaps make a test
image that's sharable, or a recipe. Basically make two 1G files of
zeros in releasedir/386 before building again and I bet that will have
trouble.)
Therefore I think that nbmakefs, at least running on i386, has some
lossage when getting over 2G. Has anyone else seen this, or know how to
fix?
Alternatively, if there's an easy way to fall back to mkisofs, that
would be a workaround - that's installed.
(I'll try to run this down and either get it fixed or file a real PR.)